The present invention relates to pressure sensitive adhesive formulations. In particular, the invention relates to a pressure sensitive adhesive formulation comprising an acrylic polymer grafted with an ethylene-butylene rubber macromer.
Typical acrylic pressure sensitive adhesive formulations are copolymers of alkyl ester monomers, a functional monomer such as acrylic acid, and may be crosslinked using, for example, aluminum chelates. These adhesives are generally deficient in adhesion to low energy surfaces. While adhesives may be tackified with rosin esters to improve low surface energy adhesion, tackification results in loss of heat resistance and poor aging properties. Even though good aging properties are compromised, tackified acrylic dispersions are sufficient for some applications, e.g. most paper label uses and, indeed, have become the dominant paper label technology. These tackified acrylic adhesives, however, do not have sufficient resistance to degradation for most graphics and industrial tape applications in which acrylic solutions are conventionally used.
Rubber-resin formulations are often used to adhere to polyolefins and other low energy substrates. Typical compositions are natural rubber or styrene block copolymers tackified With rosin esters. These formulations provide excellent tack and cohesive strength but discolor and lose tack on aging due to oxidative and UV light induced degradation. Formulations of fully hydrogenated rubbers and resins, besides being more costly, generally do not have the required adhesive performance.
U.S. Pat. No. 5,625,005 discloses hybrid rubber-acrylic pressure sensitive adhesives described as having good UV resistance and aging characteristics along with high adhesion to non-polar surfaces. Despite this advancement in the art, there remains a need for improved polymer compositions which can be used to prepare pressure sensitive adhesives having sufficient adhesion and chemical resistance properties for applications such as industrial tapes and transfer films, and exterior graphics applications on low energy, difficult to adhere surfaces. The present invention addresses this need.
The invention provides adhesive formulations having outstanding coating characteristics, adhesion to a wide variety of substrates, including low energy surfaces, while maintaining these performance properties at higher temperatures in their dried state.
One aspect of the invention is directed to a pressure-sensitive adhesive comprising an acrylic polymer grafted with a rubber macromer. Preferred for use is an ethylene-butylene macromer. In one embodiment, the acrylic polymer comprises at least one low glass transition temperature (Tg) alkyl acrylate monomer containing from about 4 to about 18 carbon atoms in the alkyl group and at least one monomer having a high glass transition temperature (i.e., a Tg greater than about 0xc2x0 C.). In preferred embodiments of the invention the acrylic polymer may further comprise at least one hydroxy functional monomer and/or may also comprise at least one carboxy functional monomer. In a particularly preferred embodiment, a crosslinking agent, such as an aluminum or a titanium crosslinking agent, is used.
Another aspect of the invention is directed to a pressure-sensitive adhesive comprising an acrylic polymer comprising at least one low Tg alkyl acrylate monomer containing from about 4 to about 18 carbon atoms in the alkyl group grafted with a rubber macromer, preferably, an ethylene-butylene macromer, the polymer being crosslinked using a titanium crosslinking agent. In a preferred embodiment, the acrylic polymer comprises, in addition to an alkyl acrylate monomer, at least one high Tg monomer, at least one hydroxy functional monomer and/or at least one carboxy functional monomer. The use of a titanium-containing metal alkoxide crosslinker has been discovered to impart excellent and unexpected high temperature performance.
Still another aspect of the invention is directed to a process of making a pressure-sensitive adhesive comprising an acrylic polymer grafted with a rubber macromer, preferably an ethylene-butylene macromer, wherein the macromer is substantially free of metal or strong acid. Preferably, the molecular weight of the macromer used to make the adhesive ranges from about 2,000 to about 10,000. The process comprises reacting an acrylic polymer component with a rubber macromer component, said macromer component being substantially free of catalyst used to prepare the macromer component.
Yet another aspect of the invention is directed adhesive articles, e.g., industrial tapes, transfer films, and the like, comprising a pressure sensitive adhesive hybrid polymer. In one particularly preferred embodiment, the hybrid polymer comprises an ethylene-butylene macromer, 2-ethylhexyl acrylate or similar low Tg acrylic monomer, methyl acrylate or similar high Tg monomer, and preferably a hydroxy functional monomer such as hydroxyethyl acrylate.
As used herein, the term xe2x80x9cpressure-sensitive adhesivexe2x80x9d refers to a viscoelastic material which adheres instantaneously to most substrates with the application of slight pressure and remains permanently tacky. A polymer is a pressure-sensitive adhesive within the meaning of the term as used herein if it has the properties of a pressure-sensitive adhesive per se or functions as a pressure-sensitive adhesive by admixture with tackifiers, plasticizers or other additives.
The adhesive polymer of the invention is a rubber-acrylic hybrid polymer comprising an acrylic polymer backbone grafted with rubber macromers including, but not limited to, ethylene-butylene macromers, ethylene-propylene macromers and ethylene-butylene-propylene macromers. In general, the hybrid polymers are made by copolymerizing alkyl acrylate ester monomers in the presence of a macromer containing a reactive acrylic or methacrylic end group. This leads to a comb-type copolymer having an acrylic backbone and side chains of macromer.
More specifically, acrylic polymer backbone contemplated for use in the practice of the invention is formed of acrylate monomers of one or more low Tg alkyl acrylates. Low transition temperature monomers are those having a Tg of less than about 0xc2x0 C. Preferred alkyl acrylates which may be used to practice the invention have up to about 18 carbon atoms in the alkyl group, preferably from about 4 to about 10 carbon atoms in the alkyl group. Alkyl acrylates for use in the invention include butyl acrylate, amyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, decyl acrylate, dodecyl acrylates, isomers thereof, and combinations thereof. A preferred alkyl acrylate for use in the practice of the invention is 2-ethyl hexyl acrylate.
The monomer system used to make the acrylic backbone polymer could be solely based on lowly Tg alkyl acrylate ester monomers, but is preferably modified by inclusion of high Tg monomers and/or functional comonomers, in particular carboxy-containing functional monomers, and/or, even more preferably, hydroxy-containing functional monomers.
High Tg monomer components which may be present, and in some embodiments are preferably present, include methyl acrylate, ethyl acrylate, isobutyl methacrylate, and/or vinyl acetate. The high Tg monomers may be present in a total amount of up to about 50% by weight, preferably from about 5 to about 50% by weight, even more preferably from about 10 to about 40% by weight, based on total weight of the hybrid polymer.
The acrylic backbone polymer may also comprise one or more functional monomers. Preferred are carboxy and/or hydroxy functional monomers.
Carboxy functional monomers will typically be present in the hybrid polymer in an amount of up to about 7% by weight, more typically from about 1 to about 5% by weight, based on the total weight of the monomers. Useful carboxylic acids preferably contain from about 3 to about 5 carbon atoms and include, among others, acrylic acid, methacrylic acid, itaconic acid, and the like. Acrylic acid, methacrylic acid and mixtures thereof are preferred.
In a particularly preferred embodiment, the acrylic backbone comprises hydroxy functional monomers such as hydroxyalkyl (meth)acrylate esters, and acrylic polymers used to form the backbone of the invention are preferably acrylic ester/hydroxy (meth)alkyl ester copolymers. Specific examples of hydroxy functional monomers include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate and hydroxypropyl methacrylate. Hydroxy functional monomers are generally used in an amount of from about 1 to about 10%, preferably from about 3 to about 7%.
Other comonomers can be used to modify the Tg of the acrylic polymer, to further enhance adhesion to various surfaces and/or to further enhance high temperature shear properties. Such comonomers include N-vinyl pyrrolidone, N-vinyl caprolactam, N-alkyl (meth)acrylamides such as t-octyl acrylamide, cyanoethylacrylates, diacetoneacrylamide, N-vinyl acetamide, N-vinyl formamide, glycidyl methacrylate and allyl glycidyl ether.
The monomer proportions of the acrylic polymer are adjusted in such a way that the backbone polymer has a glass transition temperature of less than about xe2x88x9210xc2x0 C., preferably from about xe2x88x9220xc2x0 C. to about xe2x88x9260xc2x0 C.
The macromers which may be used to prepare the graft copolymers have a glass transition temperature of about xe2x88x9230xc2x0 C. or less, preferably about xe2x88x9250xc2x0 C. to about xe2x88x9270xc2x0 C., as determined by differential scanning calorimetry (DSC), and are preferably present in an amount of from about 5 to about 50 percent by weight of the hybrid polymer. Such macromers are commercially available from Kraton Polymers Company. While the molecular weight of the macromer can range from about 2,000 to about 30,000, macromers for use in practicing the invention will preferably have a molecular weight range of from about 2,000 to about 10,000, as determined by gel permeation chromatography (GPC).
Conventionally, saturated rubber macromers may be prepared by a number of well-known methods. One method involves an anionic polymerization to produce a hydroxyl terminated conjugated diene polymer formed from, for example, 1,3-butadiene and/or isoprene monomer, as described in U.S. Pat. No. 5,625,005, the disclosure of which is incorporated herein by reference. Reduction of at least 90%, preferably at least 95%, of the unsaturation in the low molecular weight monool can be achieved through catalytic hydrogenation as taught in U.S. Pat. Nos. Re. 27,145 and 4,970,254, the disclosures of which are incorporated by reference herein. Suitable saturated rubber monools are available from Kraton Polymers Company. Kraton(copyright) L 1203 is a preferred grade. In the final step, the hydroxyl termination is reacted to form an acrylate or methacrylate group by any of a number of well known methods. These include esterification or transesterification using a strong acid or metal-containing catalyst, (e.g., compounds of Ti, Sn and the like), by reaction with an acid chloride, or via a urethane reaction employing a metal catalyst, as described in U.S. Pat. No. 5,625,005.
It has now been discovered that metal or acid residue present in the macromer used to prepare rubber acrylic hybrid polymer-based adhesives, in particular those comprising hydroxyl functional-containing polymers, can adversely affect adhesive properties. While certain low levels of metal or acid may be used for certain applications, it is preferable that the macromers be substantially free of the catalyst used in the polymerization thereof. Substantially free, as this term is defined herein, means that any catalyst residue remaining in the polymerized macromer, if any, will not cause problems in the preparation of the hybrid polymer. Removal of cataylst residue can be readily accomplished using methods well-known in the art.
The hybrid polymer of the invention may be prepared by conventional polymerization methods familiar to those of skill in the art. These methods include, without limitation, solution polymerization, suspension polymerization and bulk polymerization. In solution, the graft copolymers are synthesized by conventional free radical techniques using a solvent mixture. The solvent blend, preferably ethyl acetate, hexane and/or heptane, and toluene, imparts the solubility that is necessary for good coating behavior at low and high coat weights. In the practice of the invention, it may also be advantageous to reduce the residual monomer content following polymerization using methods which are known and conventional in the art.
The preferred adhesive compositions are preferably crosslinked using a chemical crosslinking agent. While the use of aluminum and titanium crosslinking agents may be used to practice the invention, it has been discovered that use of titanium containing metal alkoxide crosslinker is necessary for high temperature performance, and is the preferred crosslinker for hydroxyalkyl(meth)acrylate esters. The use of a titanium crosslinker imparts a yellowish color to the final product but, for many applications, is of little concern. The crosslinker is typically added in an amount of from about 0.3% to about 2% by weight of the hybrid polymer.
The adhesive compositions of this invention are preferably tackified. The acrylic and rubber components of the hybrid polymer are believed to form a microphase separated structure in the solid state. Support for this comes from the appearance of two distinct Tg""s in the temperature spectrum of viscoelastic properties corresponding to each component. Tackifying resins useful in these compositions are compatible with the rubber macromer phase. Tackifiers compatible with the acrylic phase can, of course, be used with any acrylic polymer and the hybrid polymer of this invention is no exception. However, such tackifiers are typically derived from natural rosin and are associated with poor aging characteristics. It is an objective of this invention to overcome these problems. Thus the preferred tackifiers are synthetic hydrocarbon resins derived from petroleum. Non-limiting examples of rubber phase associating resins include aliphatic olefin derived resins such as those available from Goodyear under the Wingtack(copyright) tradename and the Escorez(copyright) 1300 series from Exxon. A common C5 tackifying resin in this class is a diene-olefin copolymer of piperylene and 2-methyl-2-butene having a softening point of about 95xc2x0 C. This resin is available commercially under the tradename Wingtack 95. The resins normally have ring and ball softening points as determined by ASTM method E28 between about 20xc2x0 C. and 150xc2x0 C. Also useful are C9 aromatic/aliphatic olefin-derived resins available from Exxon in the Escorez 2000 series. Hydrogenated hydrocarbon resins are especially useful when the long term resistance to oxidation and ultraviolet light exposure is required. These hydrogenated resins include such resins as the Escorez 5000 series of hydrogenated cycloaliphatic resins from Exxon, hydrogenated C9 and/or C5 resins such as Arkon(copyright) P series of resins by Arakawa Chemical, hydrogenated aromatic hydrocarbon; resins such as Regalrez 1018, 1085 and the Regalite(copyright) R series of resins from Hercules Specialty Chemicals. Other useful resins include hydrogenated polyterpenes such as Clearon(copyright) P-105, P-115 and P-125 from the Yasuhara Yushi Kogyo Company of Japan.
The tackifying resin will normally be present at a level of 5 to 50% by weight of the adhesive composition and preferably at a level of about 10 to 40% by weight of the adhesive composition.
The formulated adhesive may also include, excipients, diluents, emollients, plasticizers, antioxidants, anti-irritants, opacifiers, fillers, such as clay and silica, pigments and mixtures thereof, preservatives, as well as other components or additives.
The pressure sensitive adhesives of the invention may advantageously be used in the manufacture of adhesive articles including, but not limited to, industrial tapes and transfer films. The adhesive articles are useful over a wide temperature range, have improved UV resistance and adhere to a wide variety of substrates, including low energy surfaces, such as polyolefins, e.g., polyethylene and polypropylene, polyvinyl fluoride, ethylene vinyl acetate, acetal, polystyrene, powder-coated paints, and the like. Single and double face tapes, as well as supported and unsupported free films are encompassed by the invention. Also included, without limitation, are labels, decals, name plates, decorative and reflective materials, reclosable fasteners, theft prevention and anti-counterfeit devices.
In one embodiment, the adhesive article comprises an adhesive coated on at least one major surface of a backing having a first and second major surface. Useful backing substrates include, but are not limited to foam, metal, fabric, and various polymer films such as polypropylene, polyamide and polyester. The adhesive may be present on one or both surfaces of the backing. When the adhesive is coated on both surfaces of the backing, the adhesive on each surface can be the same or different.